Jove’s natural satellites total nearly four score and run the gamut in size, ranging from tens to thousands of miles wide. But of these moons, four truly stand above the rest. These are Jupiter’s Galilean moons: Io, Europa, Ganymede, and Callisto. The more we learn about these surprisingly complicated worlds, first recognized as moons by Galileo Galilei himself in March 1610, the more intriguing they get. Io is the most volcanically active world in the solar system, thanks to Jupiter’s immense gravity creating tides in Io’s solid surface that reach some 300 feet (100 m) tall. Europa’s surface, on the other hand, is mostly water ice and might even hide an underground global ocean of slushy water. Ganymede, which is wider than Mercury and the largest moon in the solar system, generates its own internal magnetic field. Callisto, meanwhile, shows little evidence of recent resurfacing, which means its heavily cratered face likely preserves a record of stray detritus streaming through the early solar system.
These intriguing features are just some of the reasons the European Space Agency (with NASA as a partner) plans to launch the JUICE (JUpiter ICy moons Explorer) mission in 2022, which will see an orbiter reach the jovian system in 2029. Once there, JUICE will spend several years closely scrutinizing Ganymede, Callisto, and Europa — while also evaluating the potential of these watery worlds to harbor life.
Mass: 318 Earth masses
Equatorial diameter: 88,850 miles (143,000 km)
Average temperature: –162 F (–108 C)
Rotation period (day): 9 hours 56 minutes
Orbital period (year): 11.9 Earth years
Moons: At least 79 moons